The present invention relates to a method of determining the orientation of a surveying instrument in a borehole. More particularly, the present invention relates to a method of computing the azimuth of a surveying instrument at a particular orientation in a borehole from the measured gravitational and magnetic field data of the earth at that orientation without measuring and correcting an apparent azimuth.
A number of instruments have been described for determining the orientation of a borehole. See M. H. Haddock, Deep Borehole Surveys and Problems, McGraw-Hill Publishing (1931). One such instrument presently in use is mounted in the drill collar and utilizes three orthagonally positioned magnetometers and three orthagonally positioned accelerometers to provide voltage outputs which are proportional to the earth's gravitational and magnetic field vector components. See, for instance, U.S. Pat. Nos. 3,791,043, 3,862,499 and 4,163,324.
It is conventional practice for the results of a borehole survey to be expressed in terms of a series of values of an azimuth angle .theta. and an inclination angle .phi. taken along the length of the borehole. Those angles .theta. and .phi. are either measured or calculated from the magnetic and gravitational field data measured by the above-described accelerometers and magnetometers at a series of locations along the borehole. However, this type survey instrument is mounted in the drill string, and the drill string has its own magnetic field. See J. Chadderdon and W. T. Kittinger, "How to get the most from nonmagnetic collars for directional drilling," Oil and Gas Journal, Apr. 16, 1962, pp. 104-109. If azimuth is measured directly, the apparent azimuth must be corrected for the magnetic field of the drill string. If the azimuth angle is calculated, the magnetic field data must be corrected before that data is used to calculate the azimuth angle.
Methods for correcting that magnetic field data or the apparent azimuth angle are disclosed in U.S. Pat. Nos. 3,791,043 and 4,163,324. In brief, those methods involve an iterative calculation which corrects the apparent azimuth or the magnetic field measurements for the magnetic interference of the drill string.
Applicant has discovered that the azimuth angle can be calculated directly from the gravitational and magnetic field data without measuring the apparent azimuth and correcting that apparent azimuth for the magnetic field of the drill string. The result is that the azimuth can be calculated with a higher degree of accuracy and reliability than previously possible. The method of the present invention also lends itself to convenient computerized calculation. It is, therefore, an object of the present invention to provide a method of determining the orientation of a surveying instrument in a borehole which does not require the correction of the magnetic field data to calculate the azimuth angle of the instrument in the borehole.
It is another object of the present invention to provide a method for determining the orientation of a surveying instrument in a borehole which does not require the measurement and correction of an apparent azimuth.
It is another object of the present invention to provide a method of determining the orientation of a surveying instrument in a borehole comprising measuring the earth's gravitational and magnetic fields at a plurality of non-colinear orientations of a surveying instrument in a borehole, calculating the borehole axial magnetic component at each orientation directly from the gravitational and magnetic field measurements and computing the azimuth angle for each orientation directly from the gravitational and magnetic field measurements and the calculated borehole axial magnetic component.
It is another object of the present invention to provide a method of determining the orientation of a surveying instrument in a borehole width increased accuracy and reliability.
It is another object of the present invention to provide a method of determining the orientation of a surveying instrument in a borehole from the earth's measured gravitational field in the x, y and z axes and from the earth's measured magnetic field in the x and y axes.
A further object of the present invention is to provide a method of calculating the azimuth angle of a surveying instrument at a selected orientation in a borehole from the earth's gravitational and magnetic field data measured by that surveying instrument.
It is another object of the present invention to provide a method of calculating the inclination angle, dip and other survey data for a selected orientation of a survey instrument in a borehole from the earth's magnetic and gravitational field data measured by that survey instrument.